In order to reduce harmful exhaust gas components or improve the fuel efficiency of a spark-ignition internal combustion engine mounted on a vehicle, there have recently been proposed various types of cylinder-injection gasoline engines, in which fuel is injected directly into the combustion chamber, in place of conventional manifold-injection engines.
In cylinder-injection gasoline engines, fuel is injected from a fuel injection valve to, for example, a cavity formed on the top of a piston so that, at the time of ignition, an air-fuel mixture having an air-fuel ratio close to the stoichiometric air-fuel ratio may be produced around the spark plug. This permits firing even with an air-fuel mixture, which is lean as a whole, thereby reducing the emission amounts of CO and HC, and also greatly improves the fuel efficiency during idling or low-load travel.
In this type of gasoline engine, moreover, injection mode is switched between a compression stroke injection mode and a suction stroke injection mode in accordance with an operating state of the engine, that is, engine load. Specifically, during a low-load operation, the compression stroke injection mode in which fuel is injected mainly during the compression stroke is selected to produce a mixture with an air-fuel ratio close to the stoichiometric air-fuel ratio locally around the spark plug or in the cavity, so that satisfactory stratified combustion may take place even with an air-fuel mixture, which is lean as a whole (this injection mode is referred to as compression-lean mode). Since, in the compression stroke injection mode, engine operation is achievable even if the overall air-fuel ratio is set to a large value (e.g., to an air-fuel ratio of "40"), fresh intake air and recirculated exhaust gas (EGR) can be supplied to the cylinder in large quantities, whereby the pumping loss is reduced, thereby remarkably improving the fuel efficiency. It is, therefore, desirable that the region of engine operation in the compression stroke injection mode should be expanded as large as possible to improve the fuel efficiency.
In medium- and high-load operations, on the other hand, the suction stroke injection mode, in which fuel is injected mainly during the suction stroke, is selected to produce a mixture having a uniform air-fuel ratio throughout the combustion chamber, so that the output required during acceleration or high-speed travel can be obtained by burning a large quantity of fuel, as in manifold-injection gasoline engines (Where the air-fuel ratio is controlled to a value in the vicinity of the stoichiometric air-fuel ratio, the injection mode is called stoichiometric-feedback (S-FB) mode, and where the air-fuel ratio is controlled to a leaner side (e.g., to an air-fuel ratio of about "22") than the stoichiometric air-fuel ratio, the injection mode is called suction-lean mode).
In manifold-injection gasoline engines, the combustible air-fuel ratio zone (lean-burn zone) of the mixture supplied to the engine is narrow, and therefore, almost constant output torque is obtained in the entire combustible air-fuel ratio zone insofar as the volumetric efficiency E.sub.v is constant. Namely, the engine output torque is determined substantially uniquely by the volumetric efficiency E.sub.v. Accordingly, in manifold-injection gasoline engines, the values of engine control parameters, such as a target air-fuel ratio and target ignition timing, are set based on the volumetric efficiency E.sub.v obtained from the output of an air flow sensor, for example, and the engine operation is controlled in accordance with the control parameter values.
On the other hand, in the compression stroke injection mode control of the cylinder-injection gasoline engine, fuel is injected to the cavity at top of the piston so that stratified combustion of a lean mixture as a whole may take place, as mentioned above, and thus normal combustion is achievable if a combustible air-fuel mixture is present only around the spark plug. In other words, compared with the manifold-injection gasoline engine, the cylinder-injection engine has a much wider combustible air-fuel ratio range in terms of the overall air-fuel ratio. Specifically, in the compression stroke injection mode control of the cylinder-injection gasoline engine, engine operation is achievable over a wide air-fuel ratio range of from an ultra-lean air-fuel ratio (e.g., "50"), which is a lean-side combustible limit, to a rich-side combustible air-fuel ratio limit (e.g., an air-fuel ratio of "20"). Accordingly, even if the value of volumetric efficiency is the same, the engine output torque greatly varies with different target air-fuel ratios. The engine output torque changes nearly in proportion to the fuel supply quantity. This means that it is difficult to properly set the values of the engine control parameters, such as the target air-fuel ratio and the target ignition timing, in the compression stroke injection mode of the cylinder-injection gasoline engine in accordance with the volumetric efficiency E.sub.v.
To overcome the difficulty, the applicant hereof has proposed using, in place of the volumetric efficiency E.sub.v, an in-cylinder effective pressure P.sub.e as a parameter representative of the engine output when setting the engine control parameter values, such as the target air-fuel ratio and the target ignition timing, in the compression stroke injection mode control of the cylinder-injection gasoline engine or when determining whether or not mode switching between the compression stroke injection mode and the suction stroke injection mode is to be performed. More specifically, a target in-cylinder effective pressure (load value) P.sub.e, which is correlated with the engine output that the driver desires, is obtained based on the accelerator opening (throttle valve opening) and the engine rotational speed, and the fuel supply quantity (target air-fuel ratio), ignition timing, etc. are set in accordance with the target value P.sub.e.
In spark-ignition engines, the ignition timing is a critical factor that determines the engine output, fuel efficiency, etc. Where air-fuel mixture is burned under the same conditions, optimum ignition timing MBT (Minimum spark advance for Best Torque) for producing a maximum torque is uniquely determined. If the ignition timing shifts from the optimum ignition timing to an advance (advance angle) side or a retard (retard angle) side, then the combustion pressure cannot be effectively utilized, lowering both the output and the fuel efficiency. In cases where low-octane fuel is used, the ignition timing generally needs to be retarded from the optimum ignition timing MBT in order to prevent the occurrence of knocking or the like. Therefore, the frequency of knocking is monitored with the use of a knock sensor or the like, and while the frequency of knocking is low, the ignition timing is shifted to as close timing to the MBT as possible to thereby prevent the output or the fuel efficiency from lowering.
In cylinder-injection spark-ignition gasoline engines, a shock accompanying the switching of injection modes should desirably be prevented, and it is also desirable that the engine control at the time of mode switching be facilitated. To meet the needs, the applicant hereof has proposed controlling the engine at the time of mode switching by the method described below.
FIG. 1 shows an output torque T of the cylinder-injection gasoline engine in each of the compression-lean mode, suction-lean mode and S-FB mode as a function of ignition timing SA.
In FIG. 1, curve I indicated by the one-dot-chain line, curve II indicated by the broken line, and curve III indicated by the solid line respectively, show the engine characteristics in the compression-lean mode, the suction-lean mode, and the S-FB mode. The characteristic curves I, II, and III are plotted based on data obtained through test operations of the engine in the respective injection modes under the same conditions (the engine control parameter values such as the air-fuel ratio, and environmental parameter values such as atmospheric density are fixed). Also, in FIG. 1, sign .circle-solid. (points A, B and C) represents the ignition timing at which a torque corresponding to a mean effective pressure value XP.sub.e is obtained in the case where premium gasoline is used, and sign .DELTA. (points A1, B1 and C1) represents the ignition timing at which a maximum torque is obtained without entailing knocking in the case where regular gasoline is used.
The point A on the characteristic curve I related to the compression-lean mode (strictly, the point where a straight line (not shown) extending from the point A perpendicularly to the horizontal axis intersects with the horizontal axis) denotes ignition timing, at which a torque corresponding to a mean effective pressure equal to a switching criterion value XP.sub.e, is obtained at the time of mode switching between the compression-lean mode and another mode, and this ignition timing (point A) is nearly equal to the optimum ignition timing MBT. In the compression-lean mode, as seen from FIG. 1, ignition timing (point A1), at which a torque corresponding to the mean effective pressure XP.sub.e is obtained where regular gasoline is used, nearly coincides with the corresponding ignition timing (point A) in the case where premium gasoline is used. The ignition timing corresponding to the point A1 takes an angular value slightly retarded from that corresponding to the point A. This means that, where regular gasoline is used, knocking is less liable to occur in the compression-lean mode of the cylinder-injection gasoline engine, compared with the other injection modes, because the air-fuel mixture burns while flowing in layers along the cavity of the piston and thus the combustion gas is cooled by the wall surfaces of the cavity etc.
Similarly, points B and C on the respective characteristic curves II and III related to the suction-lean mode and the S-FB mode each denote the ignition timing, at which a torque corresponding to the mean effective pressure equal to the criterion value XP.sub.e is obtained at the time of mode switching, in the case where premium gasoline is used.
Where premium gasoline is used, the transition from the compression-lean mode to the S-FB mode may be effected at an instant when a target load P.sub.e reaches the switching criterion value XP.sub.e, and at the time of this mode switching, engine operation conditions (ignition timing, air-fuel ratio (fuel quantity), etc.) corresponding to the point A on the characteristic curve I related to the compression-lean mode may be changed to those corresponding to the point C on the curve III related to the S-FB mode, whereby the torque generated before and after the mode switching can be kept at a fixed value corresponding to the mean effective pressure XP.sub.e, permitting the injection mode switching to be effected without causing a switching shock.
Depending on the properties of fuel used in the cylinder-injection gasoline engine, however, a problem arises at the time of mode switching between the compression stroke injection mode and the suction stroke injection mode.
For example, the problem described below arises when regular gasoline, which is lower in octane number than premium gasoline, is used. Optimum ignition timings, at which no knocking occurs, in the suction-lean mode and the S-FB mode while regular gasoline is used are represented by points B1 and C1, respectively. Thus, at the time of switching the injection mode from the compression-lean mode to the suction-lean mode or the S-FB mode while regular gasoline is used, it is necessary that the ignition timing be controlled to the point B1 or C1, in order to prevent knocking. However, if, while regular gasoline is used, the compression-lean mode is switched to the S-FB mode with the other engine operation conditions (e.g., air-fuel ratio condition etc.) than the ignition timing kept unchanged before and after the mode switching, as in the case where premium gasoline is used, then the engine output torque changes from the point A1 to the point C1 in FIG. 1, producing a torque difference of .DELTA.T.sub.a. Likewise, where the suction-lean mode is switched to the S-FB mode, a torque difference of .DELTA.T.sub.b is caused. If such a torque difference is caused at the time of switching the injection modes, the driver is given a feeling of deceleration or acceleration and the drivability greatly lowers.
Referring now to FIG. 2, the reason for the occurrence of the aforementioned torque difference will be explained in more detail. FIG. 2 illustrates engine operations observed in the case where the throttle opening .theta..sub.th is varied in an incremental direction with the engine rotational speed kept fixed, wherein the generated torque T (mean effective pressure P.sub.e) is shown as a function of the throttle opening .theta..sub.th. In FIG. 2, the solid line indicates an operation line obtained where premium gasoline is used, and the broken line indicates an operation line obtained where regular gasoline is used.
Where premium gasoline is used, the compression-lean mode is selected while the engine is operated with such an engine load that the throttle opening .theta..sub.th falls within a range of zero to .theta..sub.1. If the throttle opening .theta..sub.th exceeds .theta..sub.1 and the target load P.sub.e reaches the switching criterion value XP.sub.e (point A in FIG. 2), the engine operation region shifts from the compression-lean region to the S-FB region and the injection mode is switched from the compression-lean mode to the S-FB mode. As indicated by the solid line in FIG. 2, the engine output torque takes a value (points A and A1) corresponding to the mean effective pressure XP.sub.e before and after the mode switching, and thus there occurs no torque difference at the time of mode switching.
Also in the case where regular gasoline is used, while the throttle opening .theta..sub.th falls within the range from zero to .theta..sub.1, the engine operation control is performed in the compression-lean mode by using engine control parameter values identical with those employed in the case of using premium gasoline. This is because, in the compression-lean mode, knocking does not occur even if the engine is subjected to operation control similar to that performed when premium gasoline is used.
In the S-FB region or the suction-lean region, however, the ignition timing needs to be retarded in order to prevent knocking, unlike the case where premium gasoline is used. Accordingly, in FIG. 2, when the target load P.sub.e reaches the switching criterion value XP.sub.e and the compression-lean mode is switched to the S-FB mode, the ignition timing is shifted from the point A1 to C1 in FIG. 2 and the engine-output torque changes suddenly from the point A1 to C1, producing the torque difference .DELTA.T.sub.a. Thus, where regular gasoline is used, the torque generated in the S-FB mode after the mode switching is smaller than that generated in the compression-lean mode before the mode switching by an amount corresponding to the ignition retard for prevention of knocking, unlike the case where premium gasoline is used, with the result that a shock occurs at the time of mode switching.
Also known is ignition timing control which takes account of the fact that knocking occurs differently due to individual difference of engines (variations in products) or due to change with time. Unexamined Japanese Patent Publications (KOKAI) No. 59-10862 and No. 61-157760, for example, disclose a technique of obtaining a knock learned value based on cumulative evaluation of control data etc. used in ignition retard control based on the knock sensor output, and controlling the ignition timing in accordance with the knock learned value. Even if this technique is applied to injection mode-switching type cylinder-injection internal combustion engine, however, a shock can occur at the time of switching the injection modes (engine control modes), as in the case where the ignition timing is set in the aforementioned manner in accordance with the properties of fuel.